1. Field of the Invention
The present invention relates to an image reading apparatus such as a scanner and an image process method, and particularly to an image reading apparatus and an image process method to carry out a shading correction to a read image.
2. Description of Related Art
The reading section to read an image of a document comprises a solid-state image pickup element to convert the light source to irradiate light to the document and the amount of the reflection light which is reflected by the document image into electric signal as the main component.
It is known that both sides of a document can be read simultaneously by disposing two of the reading sections at opposing positions by sandwiching the document so as to be a pair of reading sections (hereinafter, a reading section to read the front side of the document is called the first reading section and a reading section to read the back side is called the second reading section).
When the image reading apparatus reads both sides of the document, the document is irradiated by the light source and the document image is read by the solid-state image pickup element. When the light sources provided at the image reading apparatus are both lit, the pixel value becomes greater than the pixel value when the reading is carried out by either one of the first reading section or the second reading section because of the transmitted light from the surface which is the opposite side of the document reading surface. As a result, the pixel value in the high brightness side is saturated and there is a problem that the tone property of the read data is impaired.
There is disclosed a technique to avoid the influence of the transmitted light from the opposite side (JP2004-193743, hereinafter called “patent document 1”).
In patent document 1, there is disclosed a technique to improve the tone property in the high brightness side of both reading sections by setting the standard value of the shading correction in the first reading section and the second reading section so as to be the same value according to the paper quality. That is, in the example of the patent document, the standard value is set so as to be relatively high because the influence of the transmitted light is small when a thick paper is used, and the standard value is set so as to be relatively low because the influence of the transmitted light is great when a thin paper is used. In such manner, the influence of the transmitted light from the opposite side due to the paper type is avoided.
However, in the technique of patent document 1, there is a case where an appropriate shading correction is not carried out by not being able to respond to the difference caused by the property of the first reading section and the second reading section because the same standard values are set for the first reading section and the second reading section.
FIG. 17 is an enlarged diagram of proximity of the document passing surface of the first reading section, and FIG. 18 is an enlarged diagram of proximity of the document passing surface of the second readings section. FIG. 17 shows a document glass 11, a light exposure section 12 and a shading correction board 15. FIG. 18 shows a shading correction board 10 and a second reading section 22.
As shown in FIG. 17, in the first reading section, the distance L1 which is the distance between the document surface and the light exposure section 12 is longer than the distance L2 which is the distance between the light exposure section 12 and the shading correction board 15. Further, the document glass 11 shown by the shaded are in the figure is coated with an embrocation for preventing dust from attaching to the glass. The read value will be small overall because the amount of light reaching to the paper surface decreases due to the influence of the distance and the coating of the embrocation.
As shown in FIG. 18, in the second reading section 22, the distance L3 which is the distance between the document surface and the second reading section 22 is shorter than the distance L4 which is the distance between the second reading section 22 and the shading correction board 10. Therefore, the output value of high brightness is a great value. Thus, there is a possibility that the tone property of the highlighted portion of the document cannot me expressed. Further, because the second reading section 22 is a contact type image sensor, there is a possibility that the variation in the reading property due to the assembly error is great.
As described above, when the shading correction is carried out to the read values of the first reading section and the second reading section, there is a problem that an appropriate correction is not necessarily carried out even when the same standard values are used.